FIG. 6 shows a cross-sectional diagram of a conventional semiconductor device. A semiconductor chip 6 equipped in this semiconductor device is an IGBT (Insulated Gate Bipolar Transistor), MOSFET (MOS-type Field Effect Transistor), FWD (freewheeling diode), or the like. The semiconductor device equipped with an IGBT is hereinafter referred to as “IGBT device.” The semiconductor device is described below with an IGBT device as an example.
When joining an insulating circuit board 4 (a DCB board: Direct Copper Bonding board) and a metal plate 3 by solder 5 in order to assemble the IGBT device, the great difference in linear expansion coefficient between the insulating circuit board 4 and the metal plate 3 inevitably causes a phenomenon where the flat metal plate prior to the soldering process as shown in FIG. 7(a) becomes shaped into a concave curve in which the center of the rear surface of the metal plate 3 on the side opposite to the soldered insulating circuit board 4 forms the bottom after the soldering process, as shown in FIG. 7(b). Such phenomenon occurs because, as a result of joining the principal surfaces of the insulating circuit board 4 having a low linear expansion coefficient (the linear expansion coefficient of a ceramic substrate: 4.6 to 7.3×10−6K−1) and the metal plate 3 having a large linear expansion coefficient (the linear expansion coefficient of copper: 16.6×10−6K−1) to each other by the solder 5 at high temperature, the resultant IGBT device bends toward the side with a large linear expansion coefficient when the temperature returns to the room temperature.
In view of the problems associated with this warpage of the metal plate of a semiconductor device, Patent Literature 1 discloses a method for leveling the concaved metal plate by performing a shot-peening process on the rear surface of the metal plate located on the side opposite to the surface to which an insulating circuit board is joined.
Further, Patent Literature 2 describes a method for preventing the rear surface of the metal plate from warping significantly into a concave curve even when heated to be soldered, by forming a metal-ceramic joint substrate in which a metal circuit board is joined to one side of a ceramic substrate, one side of a heat sink is joined to the other side of the ceramic substrate, and a work-hardened layer is formed on the other side of the heat sink.